This invention relates to method for forming conducting, transparent coatings and, more particularly, to method for efficiently forming such coatings on hollow vitreous members and to method for forming such coatings which have a very low resistivity and which are highly transmissive.
Transparent, conductive oxide films have been known for many years with one of the earlier uses being in conjunction with heated windshields. Such films were later used extensively as electrodes for electroluminescent devices and they are finding many new applications in the lighting, electronic, and energy conservation fields. As an example, in the manufacture of so-called rapid-start fluorescent lamps which are intended to be operated at reduced wattage, it is normally necessary to provide on the inner surface of the tubular envelope, a transparent conductive coating of tin oxide or indium oxide, with the fluorescent powder coating applied thereover.
The most usual technique for applying electrically conductive and optically transparent tix oxide films is to deposit such films as tenacious coatings from stannic chloride. This consists of spraying a solution of stannic chloride on a glass surface heated to 500.degree. to 600.degree. C. The solution normally includes alcohol to prevent undesirable fuming, and dopants to increase the conductivity of the formed coating. There are certain disadvantages to the use of this compound, including reproducibility and objectionable byproducts.
Pure stannic oxide crystals are insulators, but stannic oxide films deposited by the pyrolytic decomposition of stannic chloride from solution are conductors. The conductivity of stannic oxide films deposited by the pyrolysis of stannic chloride is believed to be due to residual chlorine atoms in the lattice. In general, the chlorine concentration decreases and the oxygen deficiency in the lattice increases with increasing decomposition temperature. Further increases in conductivity of the formed films can be obtained by doping with antimony.
It has been found that the conductivity of antimony-doped tin oxide film is at an optimum at around 1% by weight antimony trioxide doping. Such doping can reduce the conductivity of the films by as much as a factor of 50.
It is known to deposit conductive tin oxide coatings by decomposing organic tin compounds, in order to form such transparent coatings for use with fluorescent lamps. As an example, Japanese Pat. No. 50-8590, dated Apr. 5, 1975, discloses depositing conducting coatings on the interior surface of a fluorescent lamp envelope by vapor deposition of an organic tin compound, which may be doped with antimony. These coatings can also be applied by vaporizing organic tin chloride compound which may be doped with antimony, using an alcohol solution. An example of such a solution is dimethyltin dichloride mixed with triphenolantimony dichloride in methanol, as disclosed in Japanese Pat. No. 42-26637, dated Dec. 16, 1967.
In Japanese Pat. No. 50-8863 dated Apr. 8, 1975 is disclosed the formation of a transparent, conducting film by heating and evaporating organo tin compounds as a major component and a small amount of organo antimony compounds which can be added as necessary with the resulting vapor reacted on the inner surface of a glass tube to form a transparent electroconductive film of tin oxide produced by the decomposition of these materials.
As indicated hereinbefore, other systems for forming the tin oxide conducting coatings are widely used, such as spraying stannic chloride onto the glass envelope which is maintained at a temperature of 500.degree. to 650.degree. C. with the use of dopants such as antimony to improve the properties thereof, see Japanese Pat. Nos. 44-4211 dated Feb. 21, 1969 and Japanese Pat. No. 39-20121 dated Sept. 16, 1964.
Very highly conductive and transparent films have been made by sputtering indium-tin oxides onto a substrate, as reported in Journal Electrochemical Society, Solid-State Science and Technology, October 1972, pp. 1368-1374.